The subject matter disclosed herein relates to an optical measurement device that measures dimensional coordinates, and in particular to a noncontact optical measurement device have multiple optical devices for measuring an object.
Noncontact optical measurement devices may be used to determine the coordinates of points on an object. One type of optical measurement device measures the three-dimensional (3D) coordinates of a point by sending a laser beam to the point. The laser beam may impinge directly on the point or on a retroreflector target in contact with the point. In either case, the instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. Optical measurement devices closely related to the laser tracker are the laser scanner and the total station. The laser scanner steps one or more laser beams to points on a surface. It picks up light scattered from the surface and from this light determines the distance and two angles to each point. The total station, which is most often used in surveying applications, may be used to measure the coordinates of diffusely scattering (noncooperative) targets or retroreflective (cooperative) targets.
The laser tracker operates by sending a laser beam to a retroreflector target that is used to measure the coordinates of specific points. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located at the center of the sphere. Since the placement of the cube corner within the sphere has a known mechanical relationship to the measured point (i.e. the perpendicular distance from the vertex to any surface on which the SMR rests remains constant, even as the SMR is rotated) the location of the measured point may be determined. Consequently, the laser tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stating this another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
One type of laser tracker contains only an interferometer (IFM) without an absolute distance meter (ADM). If an object blocks the path of the laser beam from one of these trackers, the IFM loses its distance reference. The operator must then track the retroreflector to a known location to reset to a reference distance before continuing the measurement. A way around this limitation is to include an ADM in the tracker. The ADM can measure distance in a point-and-shoot manner.
Since trackers dwell on a point, it is desirable to place a constraint on laser power to maintain a desired categorization within the IEC 60825-1 standard. Thus it is desired that the tracker work at low laser power. In addition to clearly defining the measurement point, the SMR returns a large fraction of the laser power. In contrast, a laser scanner may be arranged to move continuously, this allows a desirable IEC 60825-1 categorization since total energy deposited on a portion of a person located in the area of operation is small. Thus, laser scanners can operate at higher laser power levels and operate with non-cooperating targets, albeit typically at lower accuracy and shorter distances than a laser tracker.
The laser scanner also sends out a laser beam toward an object. Since laser trackers interact with the operator (via the retroreflector target), it is desirable for the laser to be visible. However, laser scanners may be operated at other wavelengths—for example, infrared or visible wavelengths since the operator does not need to visually see the light beam. The laser scanner receives light reflected back from the object and determines the distance to the point on the object based in part on the time of flight for the light to strike the object and return to the scanner. Some laser scanners sequentially rotate about a zenith axis and simultaneously rotating the laser beam about the azimuth axis, the coordinates for points in the area about the laser scanner may be determined. Other laser scanners direct a beam of light to a single point or in a predetermined pattern, such as a raster patter for example.
It should be appreciated that the laser scanner may obtain the coordinates for a plurality of points much faster than a laser tracker. However, the laser tracker will measure the distance with a higher accuracy. Further, since laser trackers dwell on specific points, measurements typically integrate for fractions of a second to reduce the noise in the electronics and atmospheric turbulence. Since laser scanners typically measure on the order of a million points per second, measurements are typically made in the order of microseconds or fractions of a microsecond. Thus in scanners the noise resulting from electronics and atmospheric turbulence may be much greater.
Accordingly, while existing noncontact optical measurement devices are suitable for their intended purposes the need for improvement remains, particularly providing an optical measurement device that allows an operator to select between multiple modes of operation.